Will engine meet rated RPM if boat in gear secured to the dock?

[rwidman]

I think my boat has enough power to pull the cleats out of either the dock or the boat or break the lines. It's not something I would try and I can't see a good reason for trying it.

 

[Nomad Willy]

78 Puget Trawler wrote;
"A tugboat is usually anything but under propped".
If that were true how would they get maximum pull or push .. primarily what a harbor tug is designed to do. However a tug designed to tow the Cherrios to Alaska will need maximum pull at about 10 knots. So the later will be propped more like our boats but the harbor tug will probably be propped to rated rpm at zero fwd speed. I'm really just guessing .. what do you think?

 

[smitty477]

"If that were true how would they get maximum pull or push .. primarily what a harbor tug is designed to do. However a tug designed to tow the Cherrios to Alaska will need maximum pull at about 10 knots. So the later will be propped more like our boats but the harbor tug will probably be propped to rated rpm at zero fwd speed. I'm really just guessing .. what do you think?"


Hello Eric,
Although many tugs have variable props allowing them to change their load/pitch some are still fixed pitch. For those that are fixed they are propped for a full load while towing which takes into account the heavier loads and the slower speeds. They are therefore typically 'under-propped' when they are without a load running alone.

 

[78puget-trawler]

Like I said its all about the specific job, like log towing or barge towing, need a different prop for each. Not that complicated. For a hard slog like logs, you need all the turns you can get at low speed, whereas towing barges where speed is increased, a wheel is called for that gets turns at the high end, more pitch. If you try to tow logs with a scow wheel, the engine will be lugged down and inefficient, but the log wheel wont pull as hard dragging a big ol scow around behind it. Its pitch. The term under-propped seems really out of place when referring to a tugboat that as I said typically will have a very large prop for its size. I have not worked on tugs for over 10 years but when last I did a variable pitch prop was not something usually encountered on a tug, maybe things have changed. As far as harbor work goes, I would expect it to be propped more like a log tug than a scow tug, unless it also does a lot of other tasks. If you want all your turns and don't want the engine lugging along bellowing black smoke, you gotta have a slippery prop, less pitch.